Raiders are enemies that spawn in groups at semi-regular intervals. Group size, health, skill, and equipment depend on both the set game difficulty and the time already elapsed in the game. Typically on Tough Cassandra Classic only one wounded raider spawns in the first group. The subsequent group has 3-5 raiders, and eventually, there are groups of more than 20 raiders with the lowest health being above 80.
Raiders will invoke a message when they spawn, and an alert will come when they begin their assault.

Raiders can enter "steal" mode if they see enough valued objects lying around. In this mode, they will grab your stuff and carry it off.
When they damage your colony enough, they may also be satisfied with the damage done and will decide to leave.

Alternatively, if there are downed colonists within reach, they may try and kidnap them by letting some do the actual kidnapping and some covering them. After one of your colonists has been kidnapped, a ‘ransom demand’ incident can fire, demanding silver for the colonists that has been kidnapped.

If half of the members of a humanlike raid has been killed or downed, the rest will flee in panic, where they will run as fast as they can to the edges of the map, and will not fight back, even if they are attacked.

Pirates or Outlanders can spawn with both ranged or melee weapons, all melee with personal shields, or all ranged.
Tribals usually come in a mix of ranged and melee warriors.

Raiders will attack randomly chosen constructed objects or colonists. They will not attack natural rock walls (not the case for sappers, animals (unless the animals are hostile or tamed) or unpowered turrets. They will also engage prisoners that are from an enemy faction.

A raiding party may include members equipped with melee weapons and personal shields. When a raiding party includes grenade throwers, the AI is intelligent enough to run way from grenades thrown by their own faction; this does not apply to other explosive weapons.

Counterintuitively, they can and will quite often spawn with melee weapons that result in them having a lower DPS than unarmed combat.

Also, raiders will not use the best weapon for their skills, instead randomly picking weapons. They are often seen equipped with ranged weapons when they are better at melee (completely disregarding the Brawler trait), and vice versa.

As of Alpha 14 (July 15th, 2016), the AI dressing algorithm is now better at making outfits to withstand cold temperatures. If there is no way to make a survivable outfit, pawns won’t arrive at all. No more raids arriving and keeling over in -60C weather.

Raider Types (Humanlike)

There are multiple types of Raiders in RimWorld, each with their own sets of equipment and budgets. Raider types are exclusive to their faction.

Pirates

The following is a list of raider types exclusive to Pirate bands, in order as shown in Defs:

Drifter

Drifters are effectively the cannon fodder of pirate raiders, wielding no more than a sub-par quality club, occasionally a shiv - or rarely: a knife. They are normally either naked, or seen sporting a tattered pair of pants or t-shirt also of sub-par quality. Drifters also have a 20% chance of sporting some form of crude headgear, and a 10% chance of rocking a Peg Leg for that Pirate look. Drifters are only encountered in your very first raid (or on very low difficulties), and they prove to be of very little threat. Pirates also care so little for their drifters that they don't even send them with food.

It is listed in the weapon tags that Drifters may show up with guns, but they don't appear to actually bear firearms about their person.

Scavenger

Ranked above the Drifters; Scavengers have the privilege of actually equipping firearms as opposed to paltry low-quality melee weapons. Their equipment will still generally be of bad quality - but a massive step above what Drifters have to put up with none-the-less. Scavengers are most often seen with Machine pistols, Pump shotguns, Survival rifles, and Incendiary launchers; but they can also be seen rocking Pistols, Heavy SMGs, and even LMGs. Scavengers also typically have multiple items of clothing on them, but never armoured vests.

Scavengers have a 30% chance of spawning with headwear, and a 10% chance of being hindered by a Peg Leg. Scavengers typically accompany your average second-to-fourth-or-so pirate raid, depending on wealth and difficulty; and are therefore still not a truly significant threat, but you should still be cautious at the stage of which you'll typically fight them.

Thrasher

Thrashers are a direct step up from Drifters; typically being seen with higher quality melee weapons, but they are still generally limited to clubs, shivs, and knives - with clubs still being the most common, but knives and shivs are roughly equally likely. They are also often seen wearing headgear, and they have a generous apparel budget (for their rank), but they still have a poor weapon budget which limits them to poor melee weapon types. Thrashers have an 80% chance of coming with a hat, and a 10% chance of rocking the stage with a Peg Leg. There is a chance that a Pirate Leader could show up in a raid as a Thrasher.

Thrashers are only seen early game, and are soon replaced by the advanced Slashers, so they'll generally only feature in around one or two raids under typical circumstance. They pose a minor threat, but nothing too significant.

Pirate

Pirates are a step up from Scavengers, with further improved weapon and clothing budgets. As a result, Pirates can potentially be a moderate threat to the colony should one not take sufficient care in dispatching of them. Pirates are typically equipped with higher grade firearms such as Assault rifles and LMGs; but are also often seen with Pump shotguns, Survival rifles, and Heavy SMGs, and less oft seen with Sniper rifles, and rarely Machine pistols and Pistols.

Pirates may also sport Armoured vests and Devilstrand clothing. There's a 70% chance they'll be wearing a hat, and a 10% chance they'll have a peg leg.

Outlanders

They are not strictly 'raiders' as they are often friendly and visit the colony or escort a trade caravan to your base.

Town Councilman & Town Trader ('Villager' based)

These two types of Outlanders are often found in trade caravans or as visitors, and are put together because they share the 'Villager' pawn kind. Villagers will typically be fully clothed and will have basic weapons of both the melee and firearm variety: usually clubs, knives, maces, and pistols.

Town Traders are slightly different in the fact that they can be interacted with to trade, and only 1 can spawn per caravan.

Town Guard

Town Guards are moderately powerful outlanders which can be seen accompanying Outlander Trade Caravans, or in hostile Outlander raids. Town Guards are most often seen with Pistols, but can also be seen with Machine pistols, Heavy SMGs, Pump shotguns, Survival rifles, Incendiary launchers, Sniper rifles - and even LMGs and Assault rifles.

Tribal

The following is a list of Raider types exclusive to glorified manhu - I mean tribes:

Warrior

Tribal warriors are your standard... tribal warriors! They appear in Tribal raids equipped with nothing but Clubs, and occasionally Spears, and they only wear Tribalwear - or Parkas if necessary. Individually, they're a moderate threat at most - but they can be a considerable threat and potentially overpower your defensive line due to the sheer numbers that they occur in.

Trader

Similar to the Warrior except that it can be traded with, and only 1 can spawn per caravan.

Archer

Archers are somewhat self-explanatory in their titles: they assault in masses, and let off a cloud of arrows (or pila) which could poke an eyeball or two. Archers typically utilise Pila and Great bows, but they may rarely sport Short bows too; and they wear your standard Tribalwear, and Parka if the conditions are too harsh to go without. Archers equipped with Pila and Great Bows are dangerous as a Pilum in the wrong place can render a colonist useless, or even kill them, and Great bows are effectively Wooden Survival rifles.

Chief

Chiefs are hardened, high-ranking tribals which have endured the heat of battle and the hardship of survival on the Rim, and so far made it through alive. They only get the best weapons that their tribe can possibly make, but they are ultimately glorified Archers which may have a chronic disease or few. Chiefs will only be found equipped with Pila and Great bows, of which some could be Excellent or even Masterwork quality. Chiefs are also more difficult to recruit as colonists if they survived to be imprisoned.

Mercenaries

Mercenaries are usually encountered in mid-game raids onwards, and will usually be found in Pirate raids, but can also be mixed amongst Outlander raids - but never Tribal raids. Mercenaries are high-ranking soldiers which have high budgets for weapons and clothing, and are usually encountered wearing an armored vest. There are several variations of mercenaries:

Mercenary Gunner

Mercenary gunners are the weakest mercenaries out of the lot, but still pose a moderate to considerable threat with their numbers. Gunners will spawn with pretty much any mid-range weapon with near-equal commonality: from Machine pistols to LMGs and Sniper rifles, and in-between - but never pistols. Gunners will also usually wear armoured vests, along with Kevlar helmets, Military helmets, and rarely even Power armour helmets - and they'll occasionally be seen wearing Hyperweave clothing.

Mercenary Gunners also have a 0.3% chance of spawning with a bionic body part. Mercenary gunners can also be seen with Combat Enhancing Drugs.

Mercenary Sniper

Mercenary snipers are... well, snipers! Their clothing budget is overall slightly lower than a Gunner's, but they have an improved weapon budget, and always spawn with a long-range weapon, the Sniper Rifle. Everything else remains the same.

Grenadier

Grenadiers can prove to be somewhat of a threat towards both colonist lives and structures alike. Grenadiers are identical to Mercenary gunners, except they mainly utilise grenades and molotovs, and also have a considerably lower clothing budget.

Alternatively, they may come equipped with EMP grenades, and will be called EMP grenadiers. These deal no damage but can stun turrets and down personal shields', spelling trouble for turret-based defenses or shielded melee fighters.

Mercenary Slasher

Slashers can prove to be a real threat to front-line gunmen due to their use of Personal shields and decent-quality melee weapons. A typical slasher can be overwhelmed by a couple of people beating the living daylights out of them, shooting them at point-blank range, or with concentrated fire from a distance; however, they can and will often come in large numbers, in which they can cause great trouble. Slashers have an 8% chance of spawning with bionic body parts.

While they have an average clothing budget, their usage of personal shields often leaves them with little money left for clothes, leaving some of them them to attack while half-naked.

Heavy Mercenary

Heavy Mercenaries are a force to be reckoned with in terms of firepower, and should be prioritised as a target in most combat situations. Heavy Mercenaries are specialised with heavy weaponry; and will therefore utilise Triple rocket launchers, Doomsday rocket launchers, and miniguns. Despite packing serious heat, Heavy Mercenaries have a poor clothing budget, and will therefore not be very well protected against sustained fire. Heavy mercenaries have an 8% chance of spawning with a bionic body part.

Elite Mercenary & Boss

Elite Mercenaries and Bosses are functionally identical, so the information here applies to both. Despite having a higher 'Combat Power' rating, it is arguable that Heavy Mercenaries will still pose a more significant threat than Elite Mercenaries, purely due to their explosive arsenal. However, Elite Mercenaries have exceptionally high equipment budgets, and are not to be messed with at all. They often utilise power armor helmets and devilstrand/hyperweave clothing, but never power armor. Their arsenal consists solely of LMGs, Assault rifles, and Charge rifles. Elite mercenaries have an 8% chance of spawning with a bionic body part.

Raider Types (Mechanoid)

Scyther

Scythers are fast-moving, and particularly dangerous against isolated targets due to their high shooting accuracy and their potential to instantly destroy vital organs, and risk of loss of limb - or even life - when facing one. Although Scythers are dangerous to confront at longer distances if you don't have any bionically augmented or careful marksmen, they can generally be quickly overwhelmed by a small firing squad at closer range. They always come equipped with a charge lance, which is comparable in overall firepower to a survival rifle. Scythers are also highly competent at melee, and it's generally a bad idea to send somebody in without very high quality armour, a personal shield, and a minimum of a mediocre plasteel longsword.

Centipede

Centipedes are the opposite of scythers: slow-moving, lumbering hunks of metal that can take a considerable beating to take down. As they move so slowly, it is almost feasible to pound them with mortars if possible - as opposed to risking a colonist's life and limb. Although generally of little threat to solitary targets due to forced miss radius (unless wielding an inferno cannon) due to forced miss radius, centipedes can be highly destructive against groups of people that dare step in their way. Centipedes can be equipped with miniguns, heavy charge blasters, or inferno cannons.

Raiders' Strategies

Immediate Attack

Raiders, upon either landing in their drop pods or arriving on the map sides, will proceed to instantaneously converge on your colony and attack. The best thing to do is to defend.

Mid-base Drop Pods

Raiders will drop pod right into the middle of the base, circumventing all exterior defenses.

Tribes do not have the technology to start a drop pod attack.

Immediate Attack Smart

Like Immediate Attack, but the raiders are unusually clever in their tactics because they simply steal some of your items and run. This can be devastating in early game, because a raider may steal your stockpile of components, leaving you without any means of using electricity.

Preparation

Raiders, upon either landing in their drop pods or arriving on the map sides, will proceed to stand around in a small group near-by where they spawned for a period of time and then proceed to attack. It is possible and a completely viable strategy to attack them while they are "preparing". This preparation has no effect on their attack other than the fact that it gives you more time to prepare yourself.

Sappers

Raiders, upon either landing in their drop pods or arriving on the map sides, will proceed to instantaneously converge on your colony but not attack. They will instead mine or blast a separate way into your colony away from your defenses.

This can be a fairly dangerous way (to you) for them to try and attack, especially in mountain bases which tend to have defenses concentrated at the entrances. The best thing to do is instantly assault them before they can mine too much.

On the other hand, in most open bases, they are mostly no better than regular raiders, and can be dealt with as such.

They will always construct two mortars, regardless of the number of raiders.

After constructing the amount they desire, they will proceed to bombard the colony with mortar shells, potentially causing a great deal of damage. The rest of the attackers will stand guard, protecting their makeshift mortar base.

Once enough members have been killed or downed, or the mortars are destroyed or stolen, they will proceed to converge on your colony.

During the siege, food and mortar shells will be continuously replenished upon running out through drop pods, with each containing 12 of packaged survival meals or shells.

Oddly, they rarely use the best constructors when building, meaning they sometimes fail when building their mortars. As they will not get additional components, this will make there be 1 less mortar (sometimes even 2) pounding your base.

Defense Strategies

It is recommended to keep power generation, eating areas, lighting, doors, and walls near or behind a protected entrance, as raiders tend to set fire to them, potentially causing huge damage. Raiders will prioritize firing on colonists or turrets when those colonists or turrets are firing on them, but will otherwise prioritize random objects, meaning you can put doors or walls near your defenses to temporarily distract them.

Raider Preparation

Raiders will sometimes start by standing around in a group where they spawned and will continue this until they see a colonist nearby or they hit a certain preparation time limit, at which point they begin the assault. Because a colonist can set them off early, you can plan out the time you want them to attack. It is generally best to set them off early if you're well-prepared, to avoid potentially troublesome scenarios such as having no power when their timer runs out. Conversely, you should not set them off early if your defense is not prepared. Note that raiders usually take less than one day to begin, meaning that if there is an eclipse and you rely on Solar generators, you should not wait for them and should instead set them off early while you still have some power left. Also it's good to note that raiders will remember where your traps are. This doesn't apply to other factions, though -- if, say, the Vipers of Power come in and sets off your traps, then they would remember that. But, if the Rippers of Fire later attack, they won't have that knowledge and will still set off the same traps.

Siege defense

When faced with a siege, there are a few coping strategies you can use.

You can choose to either to assault the mortar base or wait it out and repair the damage as best as you can. The choice mainly depends on the surroundings of the mortar base and your base's position. If your base is located under a mountain (your base tiles will read Overhead Mountain when you hover your cursor over them), the mortar shells won't be able to hit those tiles at all! This makes deep mining a very effective strategy against heavy bombardment. If you don't build your base into a mountain, you should at least consider digging out at least one panic room for your colonists to hide within from the shells.

If you assault their base, one possibility is sniping either the shells or the mortars, hoping an explosion kills many of the raiders. If you want to leave those intact, you can snipe the raiders themselves. Keep in mind that killing enough of them prompts them to assault your colony directly instead of continuing their siege.

A cheeky way to end a siege is to wait for the raiders to sleep, then simply uninstall and steal the mortars. This is rather hard to pull off, but can be worth it if you want some free mortars.

Mechanoid assaults

Mechanoids only come in 2 types, Scythers and Centipedes. They have much differing stats and weapons, meaning different tactics may be used.

In most raids where they come/ drop in at the edges, the Scythers will outrun the Centipedes by a great margin, giving plenty of time to deal with them before the centipedes.

Scythers

Scythers are capable of long-range sniping attacks, and are extremely deadly with melee. If fighting it from a distance, cover along with long-ranged weapons such as sniper rifles or survival rifles are vital.

Despite its high melee damage, sometimes melee fighting it may be better, as long as you have good armor and weapons.

Centipedes

Centipedes, on the other hand, specialize in crowd control and area denial; the Minigun and Heavy charge blaster can annihilate groups of colonists, while the Inferno cannon sets your colonists ablaze and burns down your base if not careful. They are incredibly durable, sporting thick armor and high health, and can take many hits before they can be downed.

Centipedes wielding the Minigun and Heavy Charge Baster can be cheesed simply by making a lone colonist, preferably with a personal shield, stand in front of your other colonists. The centipede will target that colonist, but be unable to hit it due to the forced miss radii. Watch out for friendly fire.
The Inferno cannon is not as destructive towards your colonists, but is annoying to deal with. Keep watch on your colonists at all times, and remember to send them back into cover when needed. As a precaution, build your base out of non-flammable materials to prevent large-scale fires erupting all over your colony.

Engaging it in melee is not recommended due to its armor, high health and heavy damage.

Tribal raids

Tribal raiders come in large numbers, but with relatively poor equipment; as such, it may require different strategy compared to pirate or outlander raids.

Overall, they can deal heavy damage to your colonists; they do not have good weapons, but compensate with their sheer numbers.
However, as tribalwear does not provide protection and they do not wear any form of armor, they are easier to kill individually than other raiders.

Crowd control is an important aspect in defeating tribal raids. The Minigun or LMG is an effective weapon to use as it can easily mow down groups of raiders.

Explosive weapons are also useful in crowd control.

Explosive mortars, while inaccurate, can easily destroy a sizable group of tribals at once if they hit.
Grenades can hit archers hiding behind cover, though you have to risk a colonist or two in order to even get close enough to throw them.

Their archers are dangerous; their bows can be fired from a somewhat long distance, their pila can easily kill or incapacitate a colonist, and they always come in a large volley. As with most defensive strategies, cover is essential when fighting them.

Removing rock chunks also helps in dealing with tribal raiders hiding behind them, making them much easier to hit.